This is a MIME GnuPG-signed message. If you see this text, it means that
your E-mail or Usenet software does not support MIME signed messages.
The Internet standard for MIME PGP messages, RFC 2015, was published in 1996.
To open this message correctly you will need to install E-mail or Usenet
software that supports modern Internet standards.
You could be talking about three completely different things.
1) The server returning a structurally invalid ENVELOPE response.
2) The server improperly parsed a well-formed message, and returned a
structurally valid ENVELOPE response, that incorrectly reports the MIME
structure of the message.
3) The server parsed a badly-formatted message that is faulty, in some way,
in its MIME structure. The server returned a structurally valid ENVELOPE
response, whose contents follow the "garbage in, garbage out" principle, but
you were expecting to receive a different kind of garbage.
In case 1, you should report an error -- the same error you would normally
report if the server returned an invalid response, according to your
understanding of IMAP.
In cases 2 or 3, you should accurately represent the data returned by the
server. Trying to second-guess what the message actually contains, only
leads to madness and chaos.
-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1.4.5 (GNU/Linux)
-----END PGP SIGNATURE-----